Claude review: rust: types: add `ForLt` trait for higher-ranked lifetime support

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

The most conceptually complex patch. Introduces the `ForLt` trait — a workaround for Rust's lack of higher-kinded types:

```rust
+pub unsafe trait ForLt {
+    type Of<'a>: 'a;
+
+    fn cast_ref<'a, 'b: 'a>(x: &'a Self::Of<'b>) -> &'a Self::Of<'a>;
+}
```

The `cast_ref` method enables lifetime shortening — converting `&'a T<'b>` to `&'a T<'a>` when `'b: 'a`. This is sound because `Of<'a>` is covariant in `'a`, which the proc macro enforces via a compile-time proof:

```rust
fn _covariance_proof<'__long: '__short, '__short>(
    __x: <#ident as ForLt>::Of<'__long>,
) -> <#ident as ForLt>::Of<'__short> {
    __x
}
```

The `ForLt!()` macro handles both named types and inline type expressions. The proc macro implementation (242 lines) is complex but well-structured.

**Observation:** The `cast_ref` uses `transmute` internally. The covariance proof ensures this is sound, but the proof is generated by a proc macro — if the macro has a bug that generates a trivially-satisfiable proof, soundness is compromised. The existing tests help mitigate this.

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Generated by Claude Code Patch Reviewer